Slack - A messaging app for teams who see through the Earth!
9mCollab - Collaborate (efficiently) Conclude (fast) Contribute (in the knowledge base)
chatmego - Sign in to chatmego or create an account - start connecting new friends with me, share photos, videos, messages, a story, and vote with me
Mattermost - Mattermost is an open source alternative to Slack.
Loop Email - Emailing for teams: Organize your Inbox into channels 💌
Missive - Team communication app now supporting FB, Twitter & SMS